The Maximum Tensile Stress Safety tool is based on the maximum tensile stress failure theory for brittle materials.
The theory states that failure occurs when the maximum principal stress equals or exceeds a tensile stress limit. Expressing the theory as a design goal:
The maximum tensile stress failure theory is typically used to predict fracture in brittle materials with static loads. Brittle materials include glass, cast iron, concrete, porcelain and certain hardened steels.
The design goal is to limit the greatest principal stress to be less than the material's ultimate strength in tension:
An alternate definition compares the greatest principal stress to the yield strength of the material:
For many materials (usually ductile materials), strength in compression and in tension are roughly equal. For brittle materials, the compressive strength is usually much greater than the tensile strength.
The Mohr-Coulomb theory used in the Mohr-Coulomb Stress Safety tool is generally regarded as more reliable for a broader range of brittle materials. However, as pointed out by R. C. Juvinall (Engineering Considerations of Stress, Strain, and Strength, McGraw-Hill, 1967), "There is some evidence to support its use with porcelain and concrete. Also, it has been used in the design of guns, as some test results on thick-walled cylinders tend to agree with this theory."
Options
Define the stress limit in the Details under Stress Limit Type. Use either , or , or enter a . By default, Stress Limit Type equals .
Select one of the following Stress Tool results from the Result group of the Stress Tool tab or by right-clicking and selecting > [result type]:
Safety Factor
Safety Margin
Stress Ratio
Notes
The use of a yield strength limit with brittle materials is not recommended since most brittle materials do not exhibit a well-defined yield strength.
For ductile and some other types of materials, experiments have shown that brittle failure theories may be inaccurate and unsafe to use. The brittle failure theories may also be inaccurate for certain brittle materials. Potential inaccuracies are of particular concern if the accuracy of calculated answers is suspect.
The reliability of this failure criterion is directly related to treatment of stress risers (peak stresses). For brittle homogeneous materials such as glass, stress risers are very important, and it follows that the calculated stresses should have the highest possible accuracy or significant factors of safety should be expected or employed. If the calculated results are suspect, consider the calculated stresses to be nominal stresses, and amplify the nominal stresses by an appropriate stress concentration factor Kt. Values for Kt are available in many strength of materials handbooks. For brittle nonhomogeneous materials such as gray cast iron, stress risers may be of minimal importance.
If a part or structure is known or suspected to contain cracks, flaws, or is designed with sharp notches or re-entrant corners, a more advanced analysis may be required to confirm its structural integrity. Such discontinuities are known to produce singular (that is, infinite) elastic stresses: if the possibility exists that the material might behave in a brittle manner, a more rigorous fracture mechanics evaluation must be performed. An analyst skilled in fracture analysis can use the Mechanical APDL application program to determine fracture mechanics information.
The proper selection and use of a failure theory relies on your engineering judgment. Refer to engineering texts such as Engineering Considerations of Stress, Strain, and Strength by R. C. Juvinall (McGraw-Hill) and Mechanical Engineering Design by J. E. Shigley (McGraw-Hill) for in-depth discussions on the applied theories.